The invention relates to a developing process for use in the electrophotography.
A developing process of self-bias type for use in the electrophotography is known in which in the region of a photosensitive member where a developing agent is to be supplied, a developing electrode is disposed in opposing relationship therewith with a spacing on the order of several millimeters or less. In the developing process of this type, an electric potential of the same polarity but of a reduced magnitude from that of an electrostatic latent image formed on the photosensitive member is produced on the developing electrode by either induction or through a resistor chain. The magnitude of the potential depends on the spacing between the photosensitive member and the developing electrode, the configuration of the electrode, the material used as the developing solution and the potential of the latent image on the photosensitive member. Assuming a xerographic process in which selenium is employed as a photosensitive member and is subjected to a wet developing process, a material to be copied, which is generally white with a substantial reduced proportion of image area, will yield a potential of about 50 volts substantially allover, the surface of the photosensitive member and a potential of about 20 volts on the developing electrode. A toner contained in the developing solution will be attracted toward the photosensitive member which has a higher potential than the developing electrode, so that regions of the copy which should remain white are slightly marred, causing an overall background smearing. On the contrary, where the background of the material to be copied is colored, for example, in a yellow or blue color, or, where the proportion of the image region is increased, the developing electrode may assume a potential as high as 600 volts when the potential of the latent image on the photosensitive member is on the order of 1000 volts, producing a difference in the potential therebetween on the order of 400 volts. This is low enough to cause a virtual degradation in the developing effect by producing a very light or low density copy image.
It is also known to electrically ground the developing electrode. While this scheme increases the image density, the residual potential of the photosensitive member, or the potential of regions of the photosensitive member which correspond to the white background of the material to be copied will be influenced by the surface potential of the exposed regions, again causing a background smearing.
It is also known to apply a fixed bias potential to the developing electrode, but a low bias potential results in a background smearing while a high bias potential degrades the developing effect, resulting in either one of the disadvantages mentioned above.